A derogatory for a person of african or african american decent. BUT it could also mean a generally lazy person. This term came to be when you think of african americans sitting on there porch chatting it up. Also in the movie Clerks II, the sequal to Clerks, Randal tries to reclaim it as a new term for lazy people. And thus offends several customers and his boss.
*Police Officer sees Porch Monkey 4 Life written on the back of Randal's Mooby Uniform and looks flabbergasted*